While the technique of oil well stimulation or revival through the use of explosives such as nitroglycerin is at least 120 years old, water well stimulation using this general technique is even older, and the results obtained still remain speculative in nature, with success being far from assured. This is due mainly to a lack of knowledge concerning the surrounding geological structure at the active level or "pay zone" of deep wells, and also due to difficulty in assuring use of a correct amount of explosive to enlarge the well bore and uniformly open the surrounding geological formation, rather than compacting such surrounding formation and thereby decreasing its permeability to flow. In addition, it is very desirable if the amount of debris in the well bore can be minimized to avoid expensive follow up "well bailing" procedures.
Insofar as the explosives are concerned, it has been assumed, historically, that controlled amounts of high explosive material, such as nitroglycerin and TNT can best do the job. This assumption is undoubtedly due to extensive field testing and general experience with such explosives for shallow excavation such as quarry, and road cut work.
Such assumption is found to be incorrect, however, when detonation is carried out in a deep well with little overburden movement. Here, high explosives cause the nearby rock to yield (i.e. plastic flow) and the surrounding area to severely compact and then partly unload elastically, resulting in a somewhat larger well bore cavity surrounded by a residual stress field or stress cage in which deformed rock and the fines produced by the explosion are sufficiently compressed and impermeable to seal off or severely restrict the flow of gases or liquids into or out of the surrounding formation. This result clearly frustrates the purpose of the "shoot."
By way of further explanation, the detonation pressures of most high explosives are found to be far in excess of the yield stresses of the surrounding rock and, therefore, capable of causing a substantial amount of the above-described irreversible plastic deformation of the surrounding rock.
In the area further away from the well bore, however, the amplitude of the stress wave caused by the explosion is mitigated by geometrical divergence effects and by other dissipating factors. Here the rock is initially displaced, and then tends to return to its original position. Such return is prevented, in part, by the permanently deformed area surrounding the well bore to create the above-stated region of residual stress. In the absence of such a residual stress field and containment of the explosive gases, the resulting gases would be expected to move into surrounding fractures and further extend them on a 360.degree. range into the surrounding untouched formation.
Formation of the above-described phenomenon can occur with the use of high explosives of widely varying charge sizes.
The stated problem has not been solved but has been minimized with varying degrees of success, depending upon (a) the surrounding geological formation, (b) the amount and placement of charge(s), and (c) the spontaneous opening up of leakage pathways into surrounding formations due to subsequent spontaneous break up of the newly formed stress field. The latter, of course, is not predictable or expected in all formations.
Placement of a charge below the "pay zone" and through use of the well bore itself as a gas or liquid flow pathway into the "pay zone" has provided some measure of control and predictability in well shooting, the most promising approach, however, appears to be achieved by charge shaping, coupled with the use of specialized propellant-type explosives which produce a maximum pressure less than the yield stress of the surrounding rock. Such compounds produce a flame front traveling more slowly than the speed of sound, and the underlying chemical reaction lags behind the flame front; as opposed to high energy explosives, which have a detonation wave which travels faster than sound and the bulk of the chemical energy is quickly released behind the detonation wave shock front. In both cases, the total chemical energy released is approximately equivalent or slightly less than that experienced with a propellant-type explosive.
It is an object of the present invention to obtain an explosive composition which possesses desired propellant-type characteristics and which can successfully induce multiple fractures around a selected part of a well bore hole, while minimizing well bore hole damage and formation of a residual stress field.
It is a further object to fully utilize the benefits of a propellant-type pressure pattern while maintaining the gas generating properties of a high explosive such as
(a) low peak pressures, PA1 (b) a shock energy comparable to a propellant deflagration, PA1 (c) gas formation comparable to that obtained by an explosive detonation, and PA1 (d) a substantial total energy output while still retaining cost, convenience, and packing efficiency of art-recognized high explosive compositions.
It is a still further object of the present invention to minimize formation of a residual stress field and well bore hole damage during a well shoot operation.